Electro-optic modulators are used in analog optical communications systems such as those found in cable TV or antenna remoting applications. The analog specifications lead to strict signal-to-noise ratio and spurious-free dynamic range requirements. Most known modulators do not have sufficient performance for many applications.
There are many types of modulators that convert an applied voltage related to the information signal to be transmitted into intensity modulation of an optical carrier signal. A subset is termed "mode interference modulators." These devices, such as interferometric modulators, convert some optical phase ratio between modes into intensity changes. These criteria exclude devices that directly attenuate the light such as cutoff modulators or electro-absorption modulators. Mode interference modulators include devices such as the directional coupler modulator, versions of the "TIR" modulator based on single-mode waveguides, and the mode evolution modulator.
The most commonly used modulator today is the Mach-Zehnder (MZ) interferometric modulator, see FIG. 1. An input optical carrier is coupled into an input, single-mode, optical waveguide. This light is divided between two paths, arms of the modulator, and then recombined. The relative optical phase of the carrier in the two paths is controlled by some type of optical phase modulation in one or both arms. The intensity of the light at the output is controlled by the relative phase of the carrier signal in each arm.
Most externally modulated optical analog links use a single MZ modulator that amplitude or intensity modulates an optical carrier signal in response to an applied voltage, which is related to the information bearing signal. This modulator is biased at quadrature, .O slashed..sub.1 =90.degree.. The bias point .O slashed..sub.1 is quiescent optical phase difference between the arms of the modulator. A bias point of 90.degree. gives 0 second-order distortion and maximum link electrical gain. The modulators average (dc) optical transmission is 0.5 at this bias point. This yields a link with an adequate performance in many applications.